Coating for condenser surfaces

ABSTRACT

Coatings for the heat transfer surfaces of condensers are formulated to provide wetting of the surfaces by the condensate and wicking thereon and, thus, to effect the condensation process, including condensate collection, with a minimum pressure loss. The coatings are passive in nature and include particulate silica or calcium silicate dispersed in water insoluble, noncrystalline glassy binders, particularly insolubilized potassium silicate or lead borosilicate glass.

United States Patent Paul et al. 5] Apr. 25, 1972 COATING FOR CONDENSER[56} References Cited SURFACES UNITED STATES PATENTS [721 lnvenmm Paul,Windsor; Edwin 31M", 3,091,577 5/1963 Pequignot ..203/se x warehousePoint, both of Conn- 3,206,381 9/1965 Neugebauer et a1. ..203/86 x 73A.-.-' :UlldAi t ,E tH l wgnee n e corpora ion as an PrimaryExaminer-Alfred L. Leavitt ford, Conn.

Assistant ExammerWayne F. Cyron [22] Filed: Aug. 1, 1969Attorney-Richard N. James Coatings for the heat transfer surfaces ofcondensers are for- [52] US. Cl. ..l17/l69 R, 117/129, 203/86, mu|atedto Provide wetting f the Surfaces by the condensate 202/135 R andwicking thereon and, thus, to efiect the condensation [5 1] Int. Cl..C03c 7/00 process, including condensate collection, with a minimum[58] Field of Search ..1 17/169 A, 129; 203/86; pressure loss. Thecoatings are passive in nature and include particulate silica or calciumsilicate dispersed in water insoluble, non-crystalline glassy binders,particularly insolubilized potassium silicate or lead borosilicateglass.

13 Claims, N0 Drawings COATING FOR CONDENSER SURFACES BACKGROUND OF THEINVENTION The present invention relates in general to coatings and, moreparticularly, to coatings for the heat transfer surfaces of condensersfor wetting and wicking purposes.

A measure of the efficiency of any condenser may in part be made as afunction of the pressure loss incurred therein in the condensationprocess particularly insofar as pumping power requirements areconcerned. The pressure loss can be reduced if either the condensate iscaused to form as non-adherent droplets which may easily be blown acrossthe heat transfer surfaces, or if the condensate can be caused to formas a thin, spreading film which may be easily collected.

For satisfactory non-adherent droplet formation, the surface energy ofthe heat transfer surfaces must be reduced considerably below the valueof the surface energy of the condensate and the droplet must form with acontact angle with these surfaces greater than about 135. This may beaccomplished by coating the surfaces with a hydrophobic coatingmaterialhaving a surface energy on the order of one-third that of the liquid.Suitable coatings of this general nature are described in the patent toBuckingham US. Pat. No. 2,923,640.

In order to provide spreading, thin-filrn condensation, a contact angleof the droplet with the condensing surface of about is desired. This canbe accomplished by either lowering the surface energy of the liquidbelow that of the heat transfer surfaces or by increasing the condensingsurface energy above that of the liquid. For these purposes a surfactantmay be added to the liquid or an active coating may be utilized whichreacts with and dissolves in the condensate and thereby reduces itssurface energy to a level low enough to effect the desired spread of thecondensate. Because the active coatings involve a reaction with thecondensate, they are progressively expended during the condensationprocess.

SUMMARY OF THE INVENTION The present invention relates to passivecoatings to effect wetting and wicking on the heat transfer surfaces ofcondensers.

The coatings described herein provide the desired wicking function as aresult of the chemical polarity of uncoated silica, or calcium silicate,particles dispersed throughout the coating, the silica particlesexhibiting a polar attraction for the hydroxyl ions in the condensedwater, this attractive force causing a breakdown in the condensatewhereby it is caused to form as a thin film which spreads in thedirection of adjacent silica particles. The uncoated silica is dispersedin water insoluble, noncrystalline glassy binders such as insolubilizedpotassium silicate or lead borosilicate glass.

In one preferred embodiment, a formulation comprising, by weight, 125parts silica, 12 parts zinc oxide, 222 parts potassium silicate and 500parts water is applied as a slurry to the condenser heat transfersurfaces; allowed to air dry; cured at a temperature not exceeding about500F; and activated by immersion in boiling water.

In another preferred embodiment, a formulation comprising, by weight,100 parts silica, 100 parts lead borosilicate glass frit, 5.8 partsboric acid, 5.2 parts potassium hydroxide, 3.9 parts sodium silicate and150 parts water is applied as a slurry to the condenser heat transfersurfaces; allowed to air dry; cured at a temperature not exceeding aboutI,050F; and activated by immersion in boiling water.

The coatings thus provided are thin and continuous; have good adhesionand stability to water vapor; and are characterized by excellent wettingand spreading properties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic development fromwhich the present invention evolved was directed to the provision ofwetting and wicking coatings for the nickel, aluminum and stainlesssteel alloys, particularly for use in connection with low pressure dropcondensers. In terms of basic composition, the developed coatings Waterutilize particulate silica or calcium silicate dispersed in anoncrystalline glassy binder.

For application to the nickel-base and aluminum-base alloys, a preferredinsolubilized potassium silicate coating composition is formulated asfollows:

Materials Parts by weight Silica I25 Zinc oxide 12 Potamium oxide 222Water 500 For application to the stainless steel alloys, the preferredinsolubilized potassium silicate compositions are:

Materials Parts by weight l or (2) 78 Potassium Silicate Sodiumsilicofluoride Barium silicolluoride Silica Calcium silicate Sodiumhydroxide Water Materials Parts by Weight Silica I00 Lead borosilicateglass fril (Ferro Corporation AL-2 Frit) I00 Boric acid 5.8

Potassium hydroxide 5.2

Sodium silicate 0" 3.9

In all of these formulations, a starting size of 10-75 microns isdesired for the silica, generally in a weight range'of 25-75 percentsilica on a dry basis with the vitreous enamel binder, and a weightrange of 35-75 percent silica with the potassium silicate binder. Thelower end of the silica or calcium silicate range is determined by theparticle to particle spacing required to effect a reasonable spread ofthe condensate on the wetted surfaces. Of course, maximum wicking withminimum flow and heat transfer resistance is desirable. A

suitable test to determine satisfactory wicking is to dip the edge of acoated panel into the condensate and to measure the rise of condensatethereon vertically from the surface of the condensate. A rise of atleast 1 1% inches is preferred. The upper usable limit ofparticulatesilica is determined primarily by the sufficiency of binderto provide the desired adherence.

In these formulations, the basic wetting and wicking characteristics areimparted by the uncoated silica or calcium silicate particles. Thus,these materialsare the constituents providing the desired very highsurface energy or polarity to the coating. A high 1 hydrophilic polarsurface induces a low interfacial energy between the condensate and thesurface thereby effecting the desired wetting and wicking. The potassiumsilicate or lead borosilicate glass'frit serves as the matrix or enamelin which the particulate silica is dispersed and which provides desireduniformity of coverage and adherence. In the ascoated condition, thebinder comprises a mixture of inorganic oxides providing anon-crystalline, glassy film having good adherence tothe metallicsubstrates.

The zinc oxide is added to the coating mix to convert the potassiumsilicate binder from a water soluble to a water insoluble material.Other suitable insolubilizers for potassium silicate-include thesilicofluorides (SiF of sodium, potassium, barium and magnesium, thequantity of insolubilizer in general determining the colloidalproperties of the coating. As necessary, particularly with thesilicofluoride insolubilizers, sodium hydroxide has been used as acolloidal dispersant. The vitreous enamal frit is rendered insolublesimply by heat treatment. Similarly, water, or some other readilyvaporized liquid such as alcohols or ketones, compatible with the mix,is utilized to form a slurry for ease of application to the surfaces tobe coated. In general, the more viscous slurries will provide thethicker coatings and the less viscous slurries the thinner coatings.However, satisfactory wetting and wicking are provided with very thincoatings and, therefore, the thin coatings are normally preferredparticularly insofar as they provide the minimum resistance to heattransfer in the condensation operation.

Of course the adherence of the coating is dependent to a great extentupon the cleanliness of the metallic surfaces to which they are applied,so the typical cleaning and degreasing operations are utilized toprecondition the surfaces for the coating. Furthermore, inasmuch as theadherence of inorganic coatings of this nature may be affected by thesurface finish of the substrate, the provision of a matte, or slightlyroughened,

finish is preferred. In this regard, a suitable etchant for thenickel-base alloys comprises a formulation, by volume, of 23 percentnitric acid, 4 percent hydrofluoric acid, balance water. For thestainless steel alloys, the formulation used was, by volume, 30 percentnitric acid, 3 percent hydrofluoric acid, balance water, held at 160-180F.

When coating aluminum, in addition to the preliminary cleaning andetching sequences, it has been found that an acid metal salt-typeconversion coating used as a pretreatment before application of thewetting and wicking film is a definite aid toinsolubilization of thepotassium silicate binder. Accordingly, it hasbeen the practice to firstcoat the aluminum with the conversion coating which may be chromic acidAlodine I200 (Chromate type) or aluminum phosphate Alodine 401-41(Phosphate type), which is partially sealed in water at l-l50F for 20-30seconds. If the conversion coating is utilized, the wetting coatingshould be applied within about 15 minutes of the curing operation. Theuse of conversion coatings is described in detail in Mil-C5541 AChemical Film and Chemical Film Materials for Aluminum and AluminumAlloys.

Typically, pretreated substrates have been coated by a dipping techniqueutilizing the hereinbefore described slurries. The resultant pickupcoating is allowed to air dry followed by curing for one hour at atemperature up to about 500F for coatings utilizing potassium silicateas a binder. The coatings with vitreous enamel frit binder materialrequire curing for 15-30 minutes at a temperature up to about 1,050".The cured coated articles are then immersed in boiling water for about30 minutes to activate the coating.

The invention and particular preferred embodiments thereof have beendescribed in detail in accordance with the patent statutes. It will beunderstood, however, that the specific examples are intended to beillustrative only. The invention in its broader aspects is not limitedto the exact details described, for obvious modifications will occur tothose skilled in the art.

- What is claimed is:

1. A coating for the heat transfer surfaces of a condenser to providewetting thereof by and wicking of the condensate which comprises:

a non-crystalline glassy binder adherent to the heat transfer surfacesas a thin adherent film, the binder consisting essentially of a glassformed of a mixture of inorganic oxides which are insoluble in thecondensate;

.and an inorganic wetting and wicking compound uniformly dispersed inthe binder, the compound consisting of particulate silica or calciumsilicate or mixtures thereof.

2. The coating according to claim 1 wherein:

the binder is insolubilized potassium silicate.

3..The coating according to claim 1 wherein:

the binder is a pure lead borosilicate glass.

4. The coating according to claim 3 wherein: the wetting and wickingcompound is silica.

5. The coating according to claim 3 wherein:

the condensate is water.

6. The method of improving the efficiency of water vapor removal from agas stream by condensation in a heat exchanger which comprises:

coating the heat transfer surfaces of the heat exchanger with a mixtureconsisting of particulate silica or calcium silicate dispersed in'anon-crystalline glassy binder comprising a mixture of inorganic oxidessuspended in a vaporizable dispersant;

and drying the coating and forming the binder as water insoluble,continuous film firmly adherent to the surfaces.

7. The method of providing condensate wetting and wicking in a watercondenser which comprises:

coating the heat transfer surfaces with a mixture consisting essentiallyof particulate silica, calcium silicate, or mixtures thereof, dispersedin an aqueous slurry of potassium silicate containing a potassiumsilicate insolubilizer; drying the coating;

and heat treating the coating to insolubilize the potassium silicate andform it as a thin, continuous film adherent to the surfaces.

8. The method according to claim 3 wherein:

the insolubilizer is selected from the group consisting of zinc oxideand the silicofluorides of sodium, potassium, barium, and magnesium.

9. The method according to claim 8 wherein:

the coating mixture consists essentially of, by weight, about 125 partssilica, about 12 parts zinc oxide, about 225 parts potassium silicate,together with sufficient water to form a slurry.

10. The method of providing condensate wetting wicking in a watercondenser which comprises;

coating the heat transfer surfaces with a mixture consisting essentiallyof particulate silica, calcium silicate, or mixtures thereof, dispersedin an aqueous slurry of lead borosilicate glass frit;

drying the coating;

and curing the coating and forming it as a thin, continuous filmadherent to the surfaces.

1 1. The method according to claim 10 wherein:

. the coating mixture consists essentially of; by weight, about partssilica, about 100 pans lead borosilicate glass frit, about 6 parts boricacid, about 5 parts potassium hydroxide, about 4 parts sodium silicatetogether with sufficient water to form a slurry.

12. In a method of improving the efficiency of transformation of waterfrom one fluid species into another fluid species in a heat exchanger,the steps which comprises:

coating the heat transfer surfaces of the heat exchanger with a mixtureconsisting of particulate silica or calcium silicate dispersed in anon-crystalline glassy binder comprising a mixture of inorganic oxidessuspended in a vaporizable dispersant;

and drying the coating and forming the binder as a water insoluble,continuous film firmly adherent to the surfaces.

13. In a method of improving the efiiciency of transformation of waterfrom one fluid species into another fluid species in a heat exchanger,the steps which comprises:

coating the heat transfer surfaces with a mixture consisting essentiallyof particulate silica, calcium silicate, or mixtures thereof, dispersedin an aqueous slurry of potassium silicate containing a potassiumsilicate insolubilizer or pure lead borosilicate glass;

drying the coating;

and heat treating the coating to insolubilize the potassium silicate andform it as a thin, continuous film adherent to the surfaces.

and

2. The coating according to claim 1 wherein: the binder is insolubilizedpotassium silicate.
 3. The coating according to claim 1 wherein: thebinder is a pure lead borosilicate glass.
 4. The coating according toclaim 3 wherein: the wetting and wicking compound is silica.
 5. Thecoating according to claim 3 wherein: the condensate is water.
 6. Themethod of improving the efficiency of water vapor removal from a gasstream by condensation in a heat exchanger which comprises: coating theheat transfer surfaces of the heat exchanger with a mixture consistingof particulate silica or calcium silicate dispersed in a non-crystallineglassy binder comprising a mixture of inorganic oxides suspended in avaporizable dispersant; and drying the coating and forming the binder aswater insoluble, continuous film firmly adherent to the surfaces.
 7. Themethod of providing condensate wetting and wicking in a water condenserwhich comprises: coating the heat transfer surfaces with a mixtureconsisting essentially of particulate silica, calcium silicate, ormixtures thereof, dispersed in an aqueous slurry of potassium silicatecontaining a potassium silicate insolubilizer; drying the coating; andheat treating the coating to insolubilize the potassium silicate andform it as a thin, continuous film adherent to the surfaces.
 8. Themethod according to claim 3 wherein: the insolubilizer is selected fromthe group consisting of zinc oxide and the silicofluorides of sodium,potassium, barium, and magnesium.
 9. The method according to claim 8wherein: the coating mixture consists essentially of, by weight, about125 parts silica, about 12 parts zinc oxide, about 225 parts potassiumsilicate, together with sufficient water to form a slurry.
 10. Themethod of providing condensate wetting and wicking in a water condenserwhich comprises: coating the heat transfer surfaces with a mixtureconsisting essentially of particulate silica, calcium silicate, ormixtures thereof, dispersed in an aqueous slurry of lead borosilicateglass frit; drying the coating; and curing the coating and forming it asa thin, continuous film adherent to the surfaces.
 11. The methodaccording to claim 10 wherein: the coating mixture consists essentiallyof; by weight, about 100 parts silica, about 100 parts lead borosilicateglass frit, about 6 parts boric acid, about 5 parts potassium hydroxide,about 4 parts sodium silicate together with sufficient water to form aslurry.
 12. In a method of improving the efficiency of transformation ofwater from one fluid species into another fluid species in a heatexchanger, the steps which comprises: coating the heat transfer surfacesof the heat exchanger with a mixture consisting of particulate silica orcalcium silicate dispersed in a non-crystalline glassy binder comprisinga mixture of inorganic oxides suspended in a vaporizable dispersant; anddrying the coating and forming the binder as a water insoluble,continuous film firmly adherent to the surfaces.
 13. In a method ofimproving the efficiency of transformation of water from one fluidspecies into another fluid species in a heat exchanger, the steps whichcomprises: coating the heat transFer surfaces with a mixture consistingessentially of particulate silica, calcium silicate, or mixturesthereof, dispersed in an aqueous slurry of potassium silicate containinga potassium silicate insolubilizer or pure lead borosilicate glass;drying the coating; and heat treating the coating to insolubilize thepotassium silicate and form it as a thin, continuous film adherent tothe surfaces.